1. Field of the Invention
This invention relates generally to semiconductor “Laser Diode” (LD) devices, and more specifically to broad-area “Edge-Emitting Laser” (EEL) diode and broad-area “Surface-Emitting Laser”(SEL) diode devices.
2. Description of Related Art
Semiconductor laser diodes, as coherent light sources, have been adopted for a large variety of different applications in a remarkably short amount of time; e.g., for use in “Gigabit Ethernet Local Area Network” (GELAN) applications. Almost all semiconductor laser diodes in common use today can be divided into two main design categories:
(i) “Edge-Emitting Laser” (EEL) diode designs, and
(ii) “Surface-Emitting Laser” (SEL) diode designs.
Regardless current market successes, semiconductor laser diodes, because they typically exhibit a high-degree of ‘Inhomogeneous Broadening’, have an uncertain future in several high-value market applications. This is particularly true for SEL based laser diode configurations; e.g., such as the “Vertical Cavity Surface-Emitting Laser” (VCSEL). Further, inhomogeneous broadening occurs when the environment or properties of particles in an emitting sample are non-identical. Moreover, for all semiconductor-based material, the presence of imperfections and impurities within crystalline structures alters the physical environment of the atoms that make up their crystalline structure from one lattice site to another. The random distribution of lattice point environments leads ultimately to a distribution of particles whose center frequencies are shifted in a random way throughout the crystalline lattice of semiconductor material used in the construction of laser diode devices (particularly a laser diode's gain-region), which results in an inhomogeneously broadened gain of the laser diode's gain-medium.
Additionally, when any particular semiconductor material, being crystalline in its molecular structure, has introduced to it an electromagnetic field (e.g., during electrical pumping), the crystallographic organizing molecules that comprise the semiconductor material begin to oscillate, which in turn results in the formation of acoustic packets of discrete energy (called phonons). Further, the collisions of phonons with particles that comprise a semiconductor's underlying crystalline lattice results in the perturbation for the phase of excited emissions present within same material; e.g., excited emissions such as “Spontaneous-Emission” (SE) and “Stimulated-Emission” (STE).
Consequently, when phonons collide with SE present with the semiconductor it undergoes a perturbation of its phase, which results in it becomes arbitrarily distributed relative to the laser field (stimulated emission), which in turn causes the oscillating STE undergoing amplification (via resonance) within the cavity of the laser diode, to destabilize; thereby, causing all kinds of instabilities to manifest for the laser-emission-output.
Consequently, parameters such as ‘low threshold current’ and ‘slope efficiency’ are commonly used as performance indicators in order to help laser diode designers to determine the degree of instability that is or might be present within any particular laser diode device. Further, low threshold current is a particularly important parameter to strive and watch for in semiconductor laser diodes because it reduces the total input electrical power that is not being converted into laser radiation; wherein, threshold current density depends upon the two things:
The configuration of a laser diode's resonator (i.e., via mirror and/or facet reflectivities, cavity length, confinement factor, and active-layer thickness), and
The configuration of a laser diode's gain-region and construction material used (i.e., via gain coefficient, carrier-density at transparency, carrier decay-rates). In terms of the latter, a quantum-well comprised gain-medium is found to be better (i.e., more efficient) than a bulk comprised gain-medium, while a gain-medium comprising a strain-layer quantum-well is even more efficient than an unstrained one.
Moreover, the slope efficiency, which is the laser efficiency excluding the injection power needed to achieve threshold, is typically high in semiconductor laser diodes when compared to other types of lasers. For, example quantum-well laser slope efficiencies typically equal around 50%. This translates to 1-photon produced for every 2-injection electrons, after threshold is reached. This is an impressive number, which makes semiconductor laser diodes competitive in many opto-electrical applications. It is also an interesting number because it indicates an efficient extraction of electrical power that is only possible when the laser field is able to interact with essentially the entire carrier distribution. In other words, a quantum-well comprised semiconductor gain-medium saturates more or less homogeneously, even though its band structure contributes large inhomogeneous broadening.
Moreover, please note that at high injection current semiconductor laser diodes comprising quantum-well gain-mediums show noticeable gain roll-over. Heating of the laser diode appears to be the culprit behind this particular degradation because the degree of roll-over is proportional to pulse duration, i.e. a longer pulse duration equals a lower threshold for the gain roll-over. Laser performance can also degrade with increasing ambient temperature.
Moreover, regarding the increasing ambient temperature, the degradation is specifically in the increase in lasing threshold. Therefore, we can quantify the temperature sensitivity of the semiconductor laser diode by a T0 parameter, which is given as
                                          I            th                    ⁡                      (                          T              2                        )                          =                                            I              th                        ⁡                          (                              T                1                            )                                ⁢                      exp            ⁡                          [                                                                    T                    2                                    -                                      T                    1                                                                    T                  0                                            ]                                                          (        1        )            wherein, Ith(T1) and Ith(T2) are the threshold currents at temperatures T1 and T2, respectively. At present, high T0 laser diodes tend to be configured as single “Graded-Index” (GRIN) quantum-well comprising laser diode devices. Further, the previously mentioned graded-index structure helps to capture and trap injected carriers in the active-region of the laser diode, and this is especially important at high temperature levels where the injection electrons are, on the average, much more energetic than normal. Consequently, the quantum-well structure itself also helps in increasing T0, because of its two dimensional band structure, which makes laser performance less sensitive to the changes in the carrier energy distributions with temperature, more than the three dimensional structure typically exhibited by bulk-area semiconductor gain media. Additionally, the value of T0 will vary from around 70° Celsius for bulk-area comprised semiconductor laser diodes, and to as high as over 250° Celsius for quantum-well comprised semiconductor laser diodes.
Additionally, the implication from the high slope efficiency that semiconductor laser diodes saturate homogeneously would not be surprising except that spectral data indicate differently. The spectral data for the semiconductor laser diode teaches us that increasing amounts of current injection will result in multimode emission close to threshold. Further, multimode emission results when high rates of semiconductor spontaneous-emission leads to relatively high intensities of “Amplified Spontaneous-Emission” (ASE) modes occurring below the lasing threshold of the laser diode.
While, in contrast, the spectrum for the laser diode becomes single mode at even higher current levels because of mode competition. It is interesting to note that the laser diode's spectrum reverts back to multimode emission output at even higher current levels. This multi-longitudinal mode behavior is called spectral mode hopping by those well versed in the art, and is only possible for an inhomogeneously broadened gain-medium.
Moreover, this type of behavior does not occur for VCSEL diodes. VCSEL diodes, regardless of the fact that they are “High-Q Cavity” (HQC) lasers (i.e., having a short cavity length dimension that typically equals one emission wavelength), just like all other semiconductor based laser diodes they too comprise of an inhomogeneously broadened gain-medium, but since they exhibit single longitudinal laser-emission-output the instability exhibits itself as a different kind of behavior. Moreover, for the VCSEL diode, an inhomogeneously broadened gain results instead, in an unstable state of polarity for it's laser-emission-output.
Furthermore, the polarity switching behavior occurs at different input current levels; whereby, the exhibited state of laser emission polarity undergoes a flip/flop switch like change from one particular state of linear polarity to an opposed one. This makes it practically impossible for the VCSEL diode to be used in polarity sensitive applications like magneto-optic high-density data storage.
Since most types of lasers may be unambiguously classified as being either homogeneously or inhomogeneously broadened, the dual character exhibited by the semiconductor laser diode makes its particular physics interesting and somewhat complicated. Further, in order to know the ultimate linewidth of any semiconductor laser diode we most account for fluctuations made to occur in the laser field by the presence of spontaneous-emission within the laser's resonating cavity.
The addition of a spontaneously emitted photon, which has an arbitrary phase relative to that of the laser-field, results in a random walk-off for the tip of the laser-field vector. The field amplitude remains at essentially the square of the photon number, while the phase fluctuates freely, eventually assuming all values between 0 and 2π. The diffusion of phase leads to a vanishing laser field-vector sum, and the rate of decay of the ensemble average of the field-vector is a measure of the spontaneous-emission linewidth of the laser diode. According to this picture, the laser linewidth is given by the Schawlow-Townes formula, which is given as
                              Δ          ⁢                                          ⁢                      υ                          S              -              T                                      =                  A                      n            SS                                              (        2        )            where A is the spontaneous-emission coefficient into the lasing mode and nSS is the steady-state photon number of the lasing mode.
In Addition, the linewidth of the semiconductor laser diode has a contribution that comes from fluctuations in the refractive index caused by fluctuations in the carrier-density. Further, because of gain-clamping, intensity fluctuations have negligible direct effect on the line-width; however, they do cause fluctuations in the carrier-density. Since the refractive index change due to carrier-density at the gain-peak is large in a semiconductor gain-medium, the density fluctuations cause substantial index fluctuations, which, in turn, lead to fluctuations in phase. Moreover, resulting in the following increase in the fundamental laser linewidth, which is given asΔν=(1+α2)ΔνS-T  (3)
where α is the linewidth-enhancement factor, which is a measure of the change in the medium refractive for a corresponding change in the laser diode's gain. Hence, intensity fluctuations contribute indirectly to the linewidth, even though their direct contribution is negligible. Further, in two-level media, the indirect contribution is also negligible, since the change in the index of refraction goes through zero at the gain-maximum. Zero thus multiplies the change in saturation caused by intensity fluctuations, unless the laser diode is forced to operate away from the gain-maximum.
For EEL diodes, the laser field is index guided by the diode's heterostructure in the transverse ({circumflex over (x)}) direction. Wherein, the optical guide is usually made sufficiently narrow to support only one transverse mode. Since the required guide thickness is approximately 1-μm, the transverse beam divergence may be as large as 30°. However, one should remember that if the laser field is diffraction limited in this direction, in principle it may be collimated, expanded, focused, etc. to any desired shape with conventional optics. Of course, doing so may be impractical because the needed optical elements are likely to be considerably larger than the laser diode.
In the lateral (ŷ) direction, the optical-field confinement is often weaker, leading to substantial astigmatism in the laser-emission-output. Single-mode operation is still possible with a narrow gain or index stripe width. However, for high-power operation, the lateral dimension has to be wide in order to prevent material damage due to high optical intensities. The lateral mode profile then depends more on the gain medium than is the case with the transverse mode profile, and that a wide stripe laser diode usually operates multimode. Further, the onset of multimode operation is hastened by self-focusing, which is caused by the saturation of the carrier-induced refractive index change.
Moreover, in order to promote a better understand of the self-focusing effect, we should first look at the reverse, which is where a low laser intensity profile results instead, in a lack of gain-saturation, which provides for a carrier distribution that is made to follow the injection current distribution. To put it more succinctly, when the carrier-induced refractive index is made to decrease with increasing carrier density, then the resulting refractive index distribution tends to defocus the laser field, i.e. commonly referred to as the anti-guiding effect.
Contrariwise, owing to the more typical occurrence of gain-saturation, a spatial hole is burned into the center of the distribution by the laser field. This, as a direct consequence of gain-saturation, leads to the formation of concentric variations in the refractive index distribution, which, in turn, results in the self-focusing of the laser field. Consequently, the resultant focused laser field burns a deeper hole in the carrier distribution, which, in turn, further leads to even more focusing, i.e. this is commonly referred to as the self-focusing effect. Eventually the self-focusing is balanced by diffraction and gain to provide for a final intensity profile that comprises of several narrow ‘bumps’ (i.e., commonly referred to as filaments).
Typically, filamentation makes a noise contribution that keeps laser diodes suffering from it from being utilized in current application. However, for a laser diode that has a large area gain-volume, the self-focusing effect causing filamentation, being comprised as having a very high-intensity optical field, would most likely introduce what is commonly called “Catastrophic Optical Damage” (COD) to the laser diode's molecular structure, causing it to fail entirely. Filamentation is just one instability made to occur when spontaneous-emission makes its phase perturbating contribution to resonant laser fields.
Prior art teaches hundreds of semiconductor laser diode resonators that fail to completely neutralize any one particular instability, the reason simply being, because these resonators still harbored within themselves the destabilizing phase perturbating effects contributed by spontaneous-emission. These resonator designs, some of which are described below in much detail, failed because they were designed to treat one, maybe two, particular instabilities rather than the cause of the destabilization.
In a contrariwise fashion, my “Optical Phase Conjugation Laser Diode” (OPCLD) invention, by utilizing ‘optical phase conjugation’ neutralizes the instability seeded phase perturbations that both spontaneous-emission and acoustic-phonons contribute to stimulated-emission undergoing resonant amplification. Therefore, my OPCLD invention, in stark contrast to prior art, eliminates the cause of destabilization in order to successfully effect a homogenous broadening of the gain for the semiconductor laser diode.
In the following paragraphs there is contained much relevant prior art that teaches several examples of resonator designs that effect some degree of resonance stabilization for current laser diodes. However, because these approaches fail to neutralize the arbitrary phase instability contributed to resonance by the acoustic phonons and the spontaneous-emission that occur within all semiconductor laser diodes, they fail to stabilize the amplified resonance of intracavity stimulated-emission, which is seriously degrading the performance of current laser diodes.
Take for example, the resonator of a typical VCSEL, where its gain-region is reconfigured to be physically longer along its lateral direction, while being made physically shorter along its transverse direction (e.g., forming either a rectangular or elliptical shaped gain-region), which theoretically provides more gain to one opposed polarity orientation over another. For more details, please see—Krassimir Panajotov et al., “Polarization behavior and mode structure of Vertical-Cavity Surface-Emitting Lasers with elliptical surface relief,” published in Vertical Cavity Surface-Emitting Lasers VII, Proceedings of SPIE, Vol. 4994, pp. 127-138, (2003).
Unfortunately, even after being redesigned to provide for a laser-emission-output that exhibits a stable polarity, the VCSEL still suffers from the same polarity switching instability problem it did before being redesigned. The real cause behind the VCSEL's failure to stabilize the polarity of its stimulated-emission output lies not within what is probably misconstrued as being an unfortunately redesigned laser diode resonator, but rather, lies within the arbitrary phase contribution of intracavity spontaneous-emission. For more details regarding the polarity-switching problem sometimes experienced by VCSELs. For more details, please see—R. P. van Extor, “Characterizing and understanding VCSEL polarization noise,” Proceedings of SPEE, Vol. 3946, pp 58-68, 26-28 Jan., (2000).
Furthermore, now that we have established and identified the root cause of instabilities responsible for lowing the performance of semiconductor laser diodes, lets focus more specifically upon the relevant Prior art. Prior art is filled with examples of Edge-Emitting based Laser diode designs, which are described as having a semiconductor-comprised gain-medium, for example a quantum-well semiconductor gain structure that is formed epitaxially upon an upturned surface of a semiconductor substrate wafer. Whereby, two cavity forming mirrors are created, not epitaxially grown (i.e., internal light reflecting cleaved facets form along crystalligerous striations as the result of an entire wafer being diced into individual EEL devices), when individual EELs are diced for electronic component packaging.
Generally, the two total internal reflection edge facets are positioned on opposite sides of a semiconductor comprised gain-region along angles that are perpendicular to the substrate wafer's outermost surfaces, which altogether forms a resonant cavity. Electrical and/or optical pumping of the gain-medium will generate amplified photonic radiation, which is made to propagate (i.e., intracavity photonic radiation undergoes oscillation made to build into resonantly amplified laser-emission-output) in a direction that runs parallel along the plane of the substrate wafer.
Moreover, edge-emitting laser diodes are among the most common semiconductor laser diode devices currently produced. Available commercially as individual laser diodes, laser diodes combined into a transceiver package (i.e., having a combined photo-detector and internally modulated laser diode transmitter based semiconductor comprised integrated circuit structure), and as linear-bar laser diode arrays. The linear-bar laser diode arrays are used, for example, as an optical pump source for pumping solid-state lasers in order to provide for high-power laser-emission-output levels (e.g., 1 to 100 Watts) greater than a few hundred milliwatts. Many adaptations of the edge-emitting laser will typically operate in high-order spatial modes and at multiple frequencies. This prevents their use in applications, which require high-power laser-emission output into a single transverse spatial cavity mode at a single frequency.
Furthermore, EELs exhibit a significant degree of astigmatism, and a beam aspect ratio that is generally large making it difficult to focus the beam to a small spot size preventing EELs from being used in those applications that require a focused beam output. Further, poor beam quality in EELs also makes frequency doubling of the laser-emission-output, using nonlinear optical materials, difficult and inefficient. Further, EELs, because of their much longer cavity lengths and significantly larger gain-volumes, can only be internally modulated at around 2.3-Gbits/ps before they begin to suffer significantly from dispersive pulse broadening effects, which has prevented them from being used in applications requiring a high-degree of internal modulation for the laser-emission-output transmission of data signals.
VCSELs on the other hand, due to their reduced threshold current, circular output beam, inexpensive high-volume manufacture, and high-rates of internal modulation (typically>5-Gbits per second), VCSELs are today, particularly suitable for the multimode optical fiber that typically comprises today's “Local Area Networks” (LANs). Widely adapted for LANs are the selectively oxidized VCSEL diodes, which use an oxide aperture located within its vertical cavity to produce strong electrical and optical confinement, enabling high electrical-to-optical conversion efficiency, however, a design strategy that only provides minimal modal discrimination—allowing emission into multiple transverse spatial modes.
Because, multimode configured fiber spans, such as the kind utilized in many of today's enterprise-wide LAN and Data-Center topologies, are configured to never exceed a few hundred meters in length, the multi-mode signals they carry do not undergo signal attenuation or data loss. Therefore, the typical multi-mode VCSEL diode has made an ideal coherent light source for these multi mode LAN topologies; moreover, resulting in the VCSEL capturing more than 70% total market share for semiconductor laser diodes used in Datacom applications. However, VCSELs that emit into a single transverse spatial mode are increasingly being sought-out for emerging high-value applications, including:
Data communication using single-mode optical fiber;
Barcode scanning;
Laser printing;
Optical read/write data-heads;
Long-Haul Telecommunications and Datacom transmission;
Modulation Spectroscopy; and
“Fiber to the Home” (FTTH) compliant transmitters.
However, because selectively oxidized based VCSEL diode designs exhibit a high-degree of index confinement, stable single low-order transverse spatial mode operation in selectively oxidized VCSELs is a challenging task at best. VCSELs are typically designed and constructed to have optical cavity lengths approximately one wavelength of a desired laser-emission-output (i.e., making the VCSEL a ‘High-Q Cavity’ laser diode design). This short cavity length in turn produces widely spaced resonance nodes, causing the VCSEL diode to operate within a much-desired single longitudinal optical-mode (i.e., giving the appearance of having a homogeneously broadened gain for the VCSEL).
However, because their cavity diameter dimensions (i.e., 5-μm to 20-μm) are relatively large compared to their cavity lengths, these laser diodes usually (i.e., typically the diameter size of a VCSEL's resonating optical cavity will not exceed 13-μm, which is about twelve times greater than a single wavelength of the laser-emission-output by the device) operate in multiple transverse spatial modes (i.e., generally called multimode operation); moreover, each additional transverse spatial mode possess an unique wavelength, polarization, and what is commonly called a transverse spatial profile (i.e., sometimes generally called a beam intensity pattern). For applications requiring small spot size and high spectral purity, lasing into a single transverse cavity mode, usually the lowest-order fundamental mode (i.e., TEM00), is necessary.
In general, low-order fundamental transverse cavity mode laser-emission output for a selectively oxidized VCSEL is attained by providing optical loss to higher-order transverse cavity modes. By selectively creating optical loss for the higher-order modes, we provide modal discrimination, which consequently leads to a VCSELs operation in a single transverse cavity mode. Strategies for producing VCSELs that operate in a single transverse cavity mode have been developed.
However, because these strategies are based upon either an introduction of loss being made relatively greater for higher-order cavity modes, and thereby, will provide for an increased gain for low-order fundamental transverse cavity modes. In addition, there is an alternative approach to providing loss to higher-order cavity modes, and that is to provide gain directly for the low-order fundamental transverse cavity modes instead.
Furthermore, increased modal loss provided for higher-order cavity modes has been successfully demonstrated using three different design approaches, which first includes a modal discrimination technique that uses an etched-surface relief located on the periphery of the top facet that selectively reduces the reflectivity of the top mirror for higher-order transverse optical modes. The advantage of this technique is that the ring located around the edge of the cavity, etched in the top quarter-wave mirror stack assembly can be produced during the VCSEL's initial fabrication by conventional dry-etching, or it can be post processed on a completed VCSEL die using focused ion-beam etching. A disadvantage, however, is that the etched relief requires careful alignment to the oxide aperture or it could greatly result in an increase of optical scattering loss for the fundamental transverse cavity modes, as manifested by the relatively low (i.e., less than 4-mW) single-mode laser-emission output powers that have been reported.
Consequently, it would be more desirable to introduce mode-selective loss into the VCSEL's structure during its epitaxial deposition to avoid extra fabrication steps and self-alignment problems. Two such techniques use tapered oxide current apertures and extended optical cavities within the VCSEL laser diode respectively. The first approach, pursued extensively at Sandia National Laboratories (i.e., Albuquerque N. Mex.), is predicated on designing the profile of the oxide aperture tip in order to preferentially increase loss to higher-order transverse spatial cavity modes.
Furthermore, the aperture-tip profile is produced by tailoring the composition of the “Aluminum-Gallium-Arsenide” (AlGaAs) layers, which are oxidized during fabrication to create an aperture located within the before mentioned VCSEL. Further, VCSELs containing a tapered oxide whose tip is vertically positioned at a null (i.e., node of standing wave) in the longitudinal optical standing wave will produce greater than 3-mW of single mode output, and greater than 30-dB of side-mode suppression. Creating this structure, however, requires a detailed understanding of the oxidation process, and produces additional loss for the much-desired fundamental trans-verse cavity mode.
Furthermore, one other method used to increase modal discrimination is to extend the optical cavity length of VCSEL itself and thus, increase the diffraction loss for the higher-order trans-verse spatial cavity modes. Researchers at the University of Ulm (i.e., Ulm, Germany) have reported single-mode operation up to 5-mW, using a VCSEL constructed with a 4-μm thick cavity spacer inserted within the VCSEL's optical cavity. However, the problem here is that using even longer cavity spacers can also introduce multiple longitudinal cavity modes (i.e., generally called spatial hole burning) negating the VCSEL's greatest contribution to semiconductor laser diode application, but has resulted in single transverse cavity mode operation up to nearly 7-mW. It is interesting to note that VCSELs comprising multiple wavelength cavities do not appear to suffer any electrical penalty, however, careful cavity design is required to balance the trade-offs between the modal selectivity in the transverse and spectral longitudinal cavity modes.
However, this is all rather academic, because in order to achieve stability for a laser-emission-output into a single fundamental transverse cavity mode of low-order (i.e., preferably TEM00), the required amounts of loss needed in order to discriminate the desired mode ultimately introduce so much loss that the laser-emission-output levels never exceed a few milliwatts of power for typical gain-regions 13-μm in diameter, more or less making these devices incapable of any real world application. Further, the 13-μm diameter size used for the VCSEL's gain-region typically cannot be exceeded in an attempt to increase gain because it would introduce the onset of higher-order transverse spatial cavity modes below threshold regardless of any particular loss mechanism utilized to eliminate them.
In addition, prior art also teaches that manipulating the modal gain rather than the modal loss can also produce single-mode VCSELs. One such technique was developed at Sandia National Laboratories to spatially aperture laser gain independently of the oxide aperture. The essential aspect of this VCSEL design approach is its lithographically defined gain-region, which is produced by an intermixing of quantum-well active-regions at the lateral periphery of the VCSEL's laser cavity.
Interestingly, both the modal loss and modal gain design approaches, though they have had some moderate success, do not completely resolve the multi-mode operational issue for the VCSEL because they only treat the symptoms of the instability not the cause. More succinctly, a laser diode that is capable of a high-powered single fundamental transverse cavity mode laser-emission-output, under normal resonator design conditions—like the use of conventional mirrors, is really an oxymoron. However, prior art does teach several examples of stable resonator designs that can effectively achieve a high-power laser-emission-output into a single fundamental transverse cavity mode.
In addition, prior art also teaches the use of ion implants as loss providing structures, which are formed during a VCSEL's fabrication. Wherein, the epitaxial growth of the VCSEL's bottom total-reflection “Distributed Bragg Reflector” (DBR) quarter-wave mirror-stack assembly is epitaxially grown upon a substrate wafer. After which, the VCSEL's active-region is epitaxially grown upon the top outermost surface of the previously deposited bottom total-reflection DBR (i.e., a first quarterwave mirror-stack assembly), and is typically comprised as having a minimum of one “Multiple Quantum-well” (MQW) that is sandwiched between two GRIN comprised spacer layers. After which, the VCSEL's top partial-reflection DBR (i.e., a second DBR quarter-wave mirror-stack assembly) is next epitaxially grown upon the top outermost surface of the VCSEL's MQW comprised active-region.
Moreover, prior art further teaches that upon completion of the epitaxial deposition of the various optical and/or semiconductor materials that comprise the VCSEL's MQW gain-region, the gain-region is typically homogenized via an ion-implantation process, or alternatively, is homogenized via an ion-implantation process, which is made to occur around the masked regions that were used to create the top partial-reflection DBR mirror-stack assembly. The resultant VCSEL has a central quantum-well comprised active-region structure that preferentially provides gain for a single low-order fundamental transverse spatial cavity mode.
Consequently, only a single-mode output of little more than just a few milliwatts with a side-mode suppression ratio greater than 40-dB is obtained for this approach. This approach also requires greater fabrication complexity, however, it is anticipated that higher performance can be reached with further refinement of process parameters. Further, in order to achieve a stability for a laser-emission-output into a single fundamental transverse cavity-mode of low-order (i.e., preferably TEM00), the required amounts of loss needed in order to discriminate the desired cavity mode ultimately introduces so much loss that the laser-emission-output levels never exceed only a few milliwatts of output power for a gain-region 13-μm in diameter, making these devices incapable of any real world application.
Furthermore, because of new and greater demands being made on VCSEL based applications, new types of single-mode VCSELs are currently under development at numerous laboratories around the world. These techniques introduce modal discrimination by increasing optical loss for the higher-order modes, or as an alternative introduce modal discrimination by increasing the relative gain of the fundamental optical transverse mode. A number of additional techniques not described in detail here have also been used for forcing VCSEL devices to operate in single transverse cavity modes.
Moreover, prior art further teaches techniques that include: Spatial Filtering—please see—R. A. Morgan et al., “Transverse Mode Control of Vertical-Cavity Top-Surface-Emitting Lasers,” IEEE Photon., Tech. Lett., Vol. 4, pp. 374-376, (1993), Anti-guiding Techniques—please see—Y. A. Wu et al., “Single-Mode Emission from a Passive Anti-guiding Region Vertical Cavity Surface Omitting Laser,” Electronics Lett., pp. 1861-1863, (1993), External Cavity Techniques please see—B. Koch et al., “Single-Mode Vertical Cavity Surface Emitting Laser by Graded Index Lens Spatial Filtering,” Appl. Phys. Lett., Vol. 70, pp. 2359-2361, (1997), and Altering the Top Mirror Structure—please see—H. Martinson et al., “Transverse Mode Selection in Large Area Oxide Confined Vertical Cavity Surface Emitting Lasers Using Shallow Surface Relief,” IEEE Photon. Tech. Lett., Vol. 11, pp. 1536-1538, (1999), and B. Koch et al., “Single Mode VCSEL,” Digest of the Conference on Lasers and Electro-Optics—CLEO 2000, San Francisco, Calif., pp. May 1, (2000).
Moreover, prior art further teaches that these techniques have resulted in laser-emission-output levels being limited to less than 5-mW. Apparently, a major drawback with existing VCSEL design is the performance of their laser-emission-output levels for single mode devices, which are typically very small. Further, when designers increased the lateral size (i.e., between 13-μm and 90-μm) of the VCSEL diode's gain-region in an attempt to improve the performance of laser-emission-output levels, the ending result was a multimode transverse behavior exhibited by the resultant laser-emission-output, but with significant increase in power levels.
In addition, prior art also teaches that because VCSEL diodes use epitaxially deposited multilayered quarterwave mirror-stack assemblies (i.e., sometimes-called distributed feedback reflectors) in order to provide optical feedback. These structures being constructed using optical and/or semiconductor material that are lattice-matched to the material used to construct the VCSEL's active-region.
If the VCSEL's active-region is constructed using a material regime that provides for a non-visible emission spectra (e.g., 1.3-μm to 1.6-μm for long non-visible wavelengths and 500-μm to 300-μm for short non-visible wavelengths) then the only available mirror construction material lattice-matched to a previously deposited gain-region have very low contrasting refractive indices between them, which results in the VCSEL's mirror-stack assemblies constructed using these materials of low-contrast refractive indices having almost no reflectivity. This is why the current market does not see any commercially available Indium-Phosphide based long-wavelength 1.550-μm VCSEL diodes. Further, it is quit clear that the VCSEL diode suffers from several major performance issues yet to be resolved, which limits its application to non-proprietary low-power, multimode, and visible wavelength operation.
In conventional VCSEL diodes, cavity mirrors are positioned on opposite faces of a semiconductor comprised gain-medium. A first mirror is epitaxially grown onto a semiconductor substrate. The semiconductor comprised gain-medium is epitaxially grown onto the first mirror structure. Then a second mirror is epitaxially grown onto the previously grown gain-medium.
Electrical or optical pumping generates a laser beam with laser-emission output emitted in a direction orthogonal to the plane of the substrate. Conventional VCSELs find application in optical Datacom and optical interconnect systems. VCSEL diodes are characterized by generally low-order fundamental transverse cavity mode TEM00, where laser-emission output levels above 2-mW cw degenerate into polarity switching multi-mode output emission.
In addition, prior art teaches the construction and use of a BALD based resonator design, which has either a vertically oriented (i.e., laser-emission being perpendicular to grown semiconductor layers and their growth substrate wafers) resonating optical cavity, e.g. like those used in a “Vertical Cavity Surface Emitting Laser” (VCSEL), or has a laterally oriented (i.e., laser-emission being parallel to grown semiconductor layers and their growth substrate wafers) resonating optical cavity, e.g. like those used in a EEL diode.
Furthermore, concerning the later, larger area VCSEL emitters, having large beam diameters equal too or greater than 100-am, have demonstrated laser-emission-output power levels equaling 100-mW cw to >2-W pulsed laser-emission-output. However, operation of any VCSEL diode having such a large gain-region diameter generally carries with it the penalty of a laser-emission output beam exhibiting higher-order transverse cavity modes and related multiple frequencies, and for devices having gain-region diameter sizes that exceed 1 millimeter in size suffer from the filamentation problem that ultimately leads to COD.
Furthermore, prior art also teaches the use of an external cavity VCSEL diode approach, which is commonly referred to by prior art as a “Vertical External Cavity Surface Emitting Laser” (VECSEL), whereby an external reflector serves as the output coupler. External cavity VECSEL devices can provide low-order fundamental transverse cavity mode laser-emission-output at significantly higher power levels than conventional VCSEL diodes. Previous work on external cavity vertically emitting semiconductor lasers typically resulted in low output power.
For example, the work of Sandusky and Brueck produced low laser-emission-output power and used optical pumping to excite a semiconductor gain-medium. For more details, please see J. V. Sandusky, “A cw external cavity surface-emitting laser,” Photonics Technology Letters, vol. 8, pp. 313-315, (1996). Additionally, in a study by Hadley et al., where an electrically excited VCSEL, having an external cavity configuration, produced 2.4-mW cw and 100-mW pulsed laser-emission-output into an low-order fundamental transverse cavity mode using an gain-region and emitting area equaling 120-μm. For more details, please see—M. A. Hadley, G. C. Wilson, K. Y. Lau, and J. S. Smith, “High single-traverse mode output from external cavity surface emitting laser diodes,” Applied Phys. Letters, vol. 63, pp. 1607-1609, (1993).
In addition, please see—Mooradian et al. for details regarding a VECSEL diode design called the NECSEL, which is a variation of the VECSEL approach, but is capable of producing very high-power laser-emission-output into a single fundamental transverse cavity mode. Further, the NECSEL diode design, by utilizing an extended vertical cavity surface emitting resonator, which is designed to function by striking careful balances between diffraction, the location, the size, the use of thermal lensing (i.e., caused by carrier induced change of refractive index and gain-saturation), a gain-region comprised with a large numbers of quantum-wells (i.e., greatly reduces probability of filamentation and COD), cavity length, radius of curvature for a laser-emission-output coupling mirror, can provide for a high-power laser-emission-output into a single fundamental transverse cavity mode.
For more details, please see—A. V. Shchegrov, A. Mooradian, “488-nm coherent emission by intracavity frequency doubling of extended cavity surface-emitting diode lasers,” Proceedings Of SPIE, Vol. 4994, pp. 197-205, 29-30 Jan., (2003), and A. V. Shchegrov, A. Mooradian, “Novel 980-nm and 480-nm light sources using vertical cavity lasers with extended coupled cavities,” published in Vertical Cavity Surface-Emitting Lasers VII, Proceedings of SPIE, Vol. 4994, pp. 21-31, 29-30 Jan., (2003).
Furthermore, while the NECSEL design approach is capable of producing high-power laser-emission-output into a single fundamental transverse cavity mode it suffers from two major flaws that limit its use to non-telecom and non-Datacom application (i.e., for use in optical fiber signal regenerative pumping).
Because the NECSEL uses DBR multi-layered mirror structures to provide for optical feedback the device is limited to only near infra-red and visible wavelength application.
Because the NECSEL resonator is configured to utilize internal thermal lensing (i.e., resulting from carrier induced change of refractive index and the gain-saturation that occurs for large semiconductor gain-volume) to provide for a stable resonance capable of lasing into a single fundamental transverse cavity mode at higher laser-emission output power, it cannot provide for internal modulation speeds of around 2.3-Gigabits/ps—soon to exceed the 10-Gigabits/ps that are typically being required for present and near-future low-cost fiber-optic application (e.g., such as Passive Optical Networks, Fiber-To-The-Home, and Fiber-To-The-Premises).
Moreover, to define this problem further, let us take a quick look at ion-implanted VCSELs; they typically suffer from internal modulation delays as large as 1-μsec or more; moreover, the laser turn-on delay for these devices is caused by intracavity thermal lens formation. For more details, please see—K. L. Lear, R. P. Schneider, Jr., K. D. Choquette, and S. P. Kilcoyne, Photon. Tech. Lett. 8, 740 (1996), and N. K. Dutta, L. Tu, G. Hasnain, G. Zydzik, H. Wang, and A. Y. Cho, Electron. Lett. 27, 208, (1990).
In addition, prior art also teaches many examples of narrow-stripe EEL diodes, which emit high-quality laser beams, but have several drawbacks that limit their use in high-value applications. For example, due to the inhomogeneous broadening of their gain, the practical output power for EEL semiconductor laser diodes is at best around 50-mW to 300-mW. The main reason for this is lateral mode instabilities, which arise because high output power and single mode operation have contradictory design requirements.
Moreover, for high power operation, a large optical mode cross section is needed to circumvent the material damage threshold □10 MW/cm2. While, on the other hand, one is not completely free to change the transverse (i.e., perpendicular to the plane of the p-n function) mode dimension because it is determined by the heterostructure, whose primary purpose is carrier confinement.
Furthermore, typical heterostructure only support the lowest transverse cavity modes. This leaves the lateral (i.e., parallel to the plane of the p-n junction) mode dimension as the only transverse degree of freedom. The lateral mode dimension may be increased by having a wide channel with weak lateral waveguiding. The weak optical confinement within these laser diodes, which are called broad area edge-emitting laser diodes, usually leads to multilateral mode operation. We consider multilateral mode operation to be instability because it gives rise to spectral broadening and high spatial frequencies in the lateral field distribution. Many applications are unaffected by this instability because the laser-emission-output can still be coupled to a multimode optical fiber with reasonable efficiency. However, as we expand the range of applications to include, for example, free-space communications, then it is desirable, if not necessary, for a high power semiconductor laser diode to be able to operate in a single cavity mode.
Moreover, there are several factors affecting lateral mode stability. For BALD based devices, the gain-medium plays an important role through filamentation. Filamentation results from the self-focusing due to gain-saturation. More succinctly, filamentation results when the inhomogeneous character of the laser transition and the asymmetrical electron momentum distribution of the population inversion about the peak gain-frequency lead to an appreciable carrier-induced refractive index δng that decreases with increasing carrier density.
Near threshold, where the carrier density decreases with distance from the center of the laser beam, δng causes the refractive index in gain-guided laser diodes to increase with distance from the beam center. The resulting refractive index distribution acts as a diverging lens (anti-guiding). While the net consequence of anti-guiding and diffraction is a diverging wavefront, the laser diode still has a finite steady-state beam width because of gain guiding.
Further above threshold, the higher laser peak intensity creates a dip in the carrier density because of the gain-saturation. Further, due to δng, the net refractive index has a bump in the middle. Consequently, a waveguide is created that focuses the laser beam, resulting in a higher peak intensity, which in turn creates a stronger waveguide. Further, for laser diodes, this self-focusing phenomenon is called filamentation. Wherein, a steady-state filament size is reached when the self-focusing is balanced by diffraction and carrier diffusion. Moreover, the filament is considerably narrower than the electrode width, and it has a flat wavefront, which is typical of index-guided modes.
Moreover, filamentation occurs very close to threshold, and at high excitations, the filament width will reach an asymptotic value that is independent of electrode width. The asymptotic filament size is governed by δng, carrier diffusion, and laser-wavelength (i.e., via diffraction). Further, at even higher excitations, more than one filament will appear in the lateral field distribution, which does not settle to a steady-state. The detrimental effects of filamentation are material damage due to increased peak-intensity, and increased loses via spontaneous emission due to a reduced overlap between lasing optical field and the gain-region.
In addition, prior art teaches many laser diode designs that were created to realize a semiconductor laser diode capable of emitting spatially coherent laser light (i.e., single transverse modes), and yet would still have laser-emission-output power equaling several watts to dozens of watts or more. For more details, please see Botez and Schifres, “Diode Laser Arrays,” Cambridge Press, (1994), which describes a monolithic laser diode structure that realizes a high-quality laser-emission-output and a process for producing the semiconductor comprised structure.
Furthermore, in order to remedy the drawbacks of conventional current-injection type semiconductor laser diodes, both U.S. Pat. No. 5,461,637 and U.S. Pat. No. 5,627,853 proposes that broad area surface-emitting semiconductor comprised laser diodes be optically excited by a coherent laser light source outside the cavity.
However, since these semiconductor laser diodes utilize thermal lensing to affect an increase in the refractive indices with a change in temperature—then the temperature must be increased. These semiconductor laser diode devices, however, are quit sensitive to temperature distribution, which in turn causes their spatial oscillation modes to become unstable (i.e., the instability sometimes called spatial mode hoping by those well versed in the art).
Moreover, what should be apparent by now to the reader is how difficult it is to achieve both a single low-order fundamental transverse cavity mode oscillation and a high-power laser-emission-output for conventional semiconductor laser diode devices. For example, prior art teaches that the semiconductor laser diode device describe above has a design configuration that finds it difficult to emit laser light with higher output power into a single transverse cavity mode.
For more details, please see—Nakamura et al., “InGaN/GaN/AlGaN—Based Laser Diodes Grown on GaAs substrates with a Fundamental Transverse Mode,” Japanese Journal of Applied Physics, Part 2 Letters, vol. 37, pp. L1020, (1998), which discloses an InGaN based short-wavelength semiconductor laser diode device. Additionally, prior art further teaches a quality high-powered laser-emission-output semiconductor laser diode; described in great detail in an abstract written by B. Pezeshki et al., “400-mW Single-Frequency 660-nm Semiconductor Laser,” published in IEEE Photonics technology Letters, vol. 11, pp. 791, (1999), which discloses an AlGaInP based (i.e., which comprises of material capable of visible spectra emission wavelengths) semiconductor laser diode design.
In addition, prior art teaches a BALD design approach to solve the previously described problems of unstable low-power multimode laser-emission-output, which includes what is sometimes called a phased array semiconductor laser diode. Further, the BALD design approach comes in two flavors, including:
(i) Edge-Emitting phased array semiconductor laser diodes, and
(ii) Surface-Emitting phased array semiconductor laser diodes.
Moreover, an Edge-Emitting Phased Array semiconductor laser diode is configured to have multi-emission or broad area emission capabilities, and in particular to a phased array laser diode or a phased locked array laser diode having preferred fundamental supermode operation with a structural design that utilizes “Impurity Induced Disordering” (IID).
Furthermore, prior art teaches that an Edge-Emitting phased array semiconductor laser diode comprises a plurality of closely coupled or spaced emitters on the same integral structure or substrate. Examples of such phased array laser diodes have been thoroughly described and illustrated in U.S. Pat. No. 4,255,717, also in Pat. No. Re. 31,806, and also described in an article by William Streifer et al., entitled “Phased Array Diode Lasers,” published in the June, (1984), issue of ‘Laser Focus World’ and ‘Electro-Optics’ magazines. Wherein, the laser emitters of such laser diodes are represented by a periodically spaced current confinement means (e.g., contact stripes used for current pumping and to establish spaced optical cavities in the active gain-region of these laser diodes).
Moreover, prior art continues by teaching that the current confinement means may be interconnected or closely spaced to a degree that the optical mode established in each of the lasing cavities below a respective current confinement will couple to its neighboring optical mode, i.e. the evanescent waves will overlap into adjacent optical lasing cavities. Further, the array of optical fields produced, become locked in phase, and if the phase difference between the adjacent current confinement equals zero, then the lateral radiation pattern in the far field will comprise a much-desired single lobe.
However, prior art also teaches that a phased array laser diode will not operate in a single mode, but rather operates with two or more lobes in a far field pattern. Consequently, the phase relationship between adjacent optical modes is not under independent control and the phases will adjust themselves in a manner that minimizes laser threshold current.
In most cases, it appears that the lasing mode favored is a supermode, which results as a direct consequence of the optical fields located between adjacent optical emitters pass through zero. This is because in most real world index-guided laser diodes, as well as in many gain-guided laser diodes, pumping is reduced for locations that lay between laser emitters requiring overall reduced current pumping.
Furthermore, the prior art teaches that the foregoing explanation can be exemplified as follows. An array laser diode with Nth coupled emitters has Nth possible coupled modes, which are referred to as “supermodes.” A supermode is a cooperative lasing of the Nth-optical emitters or filaments of the array laser diode. Since there are Nth optical emitters, there are Nth possible supermodes, because all these emitters are optically coupled.
Each supermode has the property that the 1st and the Nth supermode have the same intensity pattern or envelope, the 2nd and the (N−1)th have the same intensity envelope, and in general, the ith and (N−i)th have the same intensity envelopes. The 1st or fundamental supermode has all emitters lasing in phase with an amplitude distribution representative of half a sinusoidal cycle. This is the only supermode pattern that radiates in a single central lobe in the far field pattern because all emitters are in phase.
Therefore, for a uniformly spaced array of identical emitters, the 1st and Nth supermode envelopes are half a sinusoidal period, the second and the (N−1)th supermode envelopes are two half-sinusoidal periods, etc. The phase relationship between the individual emitters in Nth supermodes differs. More specifically, for the 1st supermode, all emitters are in phase, and for the Nth supermode, the phases alternate between zero and π.
Usually the 1st and Nth supermodes have the lowest current thresholds as compared to all other supermodes because their intensity envelopes do not exhibit nulls near the center of the array where the charge density is greater because of current spreading and charge diffusion in the active region of the laser diode array. However, as previously indicated, the Nth supermode, which radiates in two lobes, has a lower current threshold of operation than the 1st supermode. Further, phased array laser diodes have a high utility due to their high power output. It is preferred that the power be concentrated into a single lobe, i.e. in the 1st supermode. The reason being is that a substantial majority of laser applications require power in a single far field lobe. Further, if lasing is experienced in more than one lobe, measures are taken to diminish or otherwise attempt to eliminate or block off the other operating lobes in the far field pattern.
Moreover, prior art further teaches that there has been much activity relative to phase locked array laser diodes or phased array laser diodes, where efforts have been established to discriminate among the supermodes to provide for fundamental supermode selection. One such suggestion was at the IEEE 9th Conference in Brazil, July 1984, where J. Katz et al. presents a talk on supermode discrimination via a controlled lateral gain distribution along the plane of the lasing structure by incorporating a separate contact to each laser diode array structure, and tailoring the currents through the array laser diode structures themselves. Further, the abstract presented for the talk can be found in the Proceedings of the Conference—pages 94 and 95, in a document entitled “Supermode Discrimination in Phase-Locked Arrays of Semiconductor Laser Arrays.”
Furthermore, more recently are the articles of Twu et al. entitled “High Power Coupled Ridge Waveguide Semiconductor Laser Arrays,” Applied Physics Letters, Vol. 45(7), pp. 709-711, Oct. 1, (1984), and S. Mukai et al. entitled “Fundamental Mode Oscillation of Buried Ridge Waveguide Laser Array,” Applied Physics Letters, Vol. 45(8), pp. 834-835, Oct. 15, (1984). Wherein, these articles suggest discrimination among the supermodes to obtain the single lobe fundamental supermode by employing index guided ridge waveguide structure, where the laser diode structures are uniformly pumped with an optical field mainly confined to the ridge region of the structure, while higher gain is experienced in the valley or coupling regions to induce in-phase operation (0° phase) and promotion of fundamental supermode operation.
In addition, prior art also teaches a similar techniques used to discriminate among supermodes, which is disclosed in U.S. patent application Ser. No. 667,251, which was filed Nov. 1, 1984. Further, prior art also teaches a technique proposed in U.S. Pat. No. 4,624,000, entitled “Phased Array Semiconductor Lasers With Preferred Emission in a Single Lobe.”, which relates to the use of structural means associated with the laser diode to enhance the amount of gain experienced in regions between adjacent optical cavities of lasing structures by spatially modulating the optical overlap of the optical field of each of the laser structures across the entire array to thereby favor the fundamental supermode over other potential modes.
In addition, prior art also teaches a phased array semiconductor laser diode design approach that includes a sizable array of VCSELs, which was originally disclosed by Jewell et al. in U.S. Pat. No. 4,949,350. Further, the patent first describes the growth of a vertical-cavity Fabry-Perot resonator structure on a substrate as being laterally undefined. Vertically, it consisted of upper and lower interference mirrors separated by an optical distance equal to the lasing wavelength. The mirrors are further described as sandwiching an active layer comprising of several quantum-well layers, which emitted light at the lasing wavelength when current was passed through them.
All the layers were described as being constructed using III-V based semiconductors that were epitaxially deposited by “Molecular Beam Epitaxy” (MBE) on a doped GaAs substrate wafer. Wherein the layers deposited above the active-region were p-type, while those deposited below were n-type to form a laser diode. A layer of gold was then deposited over the upper mirror as a contact layer.
Moreover, the laser diode array was then laterally defined by a photolithographic definition of a nickel mask above the intended lasers followed by chemically assisted, ion-beam etching. The ion-beam etching was carried through the entire vertical-cavity structure to create an array of pillars having heights of more than 5-μm. Each pillar was a separate laser and was electrically selected by contacting the metal at the top of the respective pillar. The conductive substrate served as a common counter electrode. Light was emitted through the substrate. Jewell et al. demonstrated their invention with lasers having diameters ranging down to 2-μm. Thus, it became possible to fabricate extremely dense arrays of lasers.
Moreover, the pillar lasers of Jewell et al. suffer from several problems, e.g. electrical contacts need to be formed onto the top of the high aspect-ratio pillars, which posses a fabrication problem. Additionally, for small pillar laser diodes, the relatively large sidewalls cause excessive recombination. Consequently, heat cannot be efficiently dissipated from the pillar laser structures, causing degradation in laser-emission-output performance. The sophisticated processing of Jewell et al. raises questions of manufacturability.
For example, Jewell et al. suggests that planarization using polyimide would maintain the index-guide optical waveguiding function and current confining function of the previously defined pillars and yet, would ease the contacting problem. Work is progressing on this approach and on regrowth using insulating AlGaAs, which would help solve the recombination and thermal dissipation problems, but currently the results are not very satisfactory.
In addition, prior art further teaches a planarized array of vertical-cavity surface-emitting lasers, which is disclosed by Orenstein et al. in U.S. patent application, Ser. No. 480,117, which was filed on Feb. 14, (1990); Also, disclosed in an abstract entitled “Lateral definition of high performance surface emitting lasers by planarity preserving ion implantation processes,” published in Conference Proceedings, CLEO, pages 504-505, May 21-25, (1990); and Also, in an abstract entitled “Vertical-cavity surface-emitting InGaAs/GaAs lasers with planar lateral definition,” Applied Physics Letters, volume 56, pages 2384-2386, (1990).
Wherein, Orenstein et al. constructed the same vertical cavity surface-emitting phase locked structure as in the Jewell et al. phased array. However, they performed the lateral definition via an ion implanting of protons in regions surrounding the intended laser pillars, which extended down to just above the active-region layer. The protons reduced the conductivity of the implanted region; thus, current was successfully gain-guided through the laser's gain-region.
However, using the above-mentioned approach Orenstein et al. might have retained the current gain-guiding of Jewell et al., but as a result sacrificed any index guiding advantages, since the protons did not have a significant effect on the refractive index of the implant area. Consequently, the deep ion implantation of their technique places a lower limit on the size of the lasers and the separation between adjacent laser diode pillars.
Although, VCSELs provide the advantage of lasers having very small areas and low threshold current some applications require higher optical power. In principle, a SEL can achieve high-power through a simple increase in the cross-section of the lasing gain-region with a constant current density. Recent experiments, however, has demonstrated that this technique does not work.
For larger sized surface-emitting lasers, the produced laser light is filamented into irregularly and perhaps separated lasing areas. Similar filamentation has also been observed in edge emitting “Broad Area Laser Diodes” (BALD), which, for both SEL diodes and BALD devices, is due to inhomogeneities in the gain and resultant refractive index distributions of their optical waveguides, the reasons being previously explained.
Moreover, for VCSELs, filamentation can additionally arise from spatial variations in mirror reflectivities, which must be necessarily, above 99% because of their short gain lengths and high cavity finesse. The previously mentioned spatial variations are enough to induce lasing preferentially in some regions but not in others. Aside from efficiency and thermal problems, the sparsely connected filaments are not likely to be phase-locked or even to have the same frequency. Therefore, a large area surface-emitting laser tends to lose its laser characteristics. Further, even medium sized lasers (i.e., lasers having gain-regions 5-μm to 40-μm in diameter) are bound to oscillate in a large number of transverse cavity modes, the distribution of which is uncontrollable.
Moreover, prior art further teaches that a Yoo et al. has disclosed an array of small phase-locked lasers in a abstract document entitled “Fabrication of a two-dimensional phased array of vertical-cavity surface-emitting lasers,”, which was published in Applied Physics Letters, volume 56, pages 1198-1200, (1990). In this refinement of the Jewell et al. technique, they fabricated a rectangular array to have more than 160 laser diodes configured within a 25-μm2 surface area. Each laser diode structure had a square dimension equaling 1.3-μm2, and is separated from neighboring laser diodes by a space of no less than 0.1-μm. The circular array was planarized with polyimide and a common upper electrode attached to all the LDs located within the array. The angular distribution of the far-field optical intensity showed substantial, though possibly not complete, phase locking between all of the laser diodes.
Moreover, Yoo et al. was able to achieve phase locking between the strongly waveguiding of the Jewell et al. pillars design, but only by a very small separation between the pillars and the small areas of the pillars themselves. The calculations of Yoo et al. in “Array Mode Analysis of Two-Dimensional Phased Arrays of Vertical Cavity Surface Emitting Lasers,” IEEE Journal of Quantum Electronics, volume 26, pages 1039-1051, (1990), have shown this requirement of small laser spacing for strongly waveguided structures. However, such a structure and associated processing produce very high surface recombination on the sides of the pillars because of the large surface-to-volume ratio. As a result, their phase-locked array showed poor efficiency and threshold current, and their phase locking was not complete.
Additionally, prior art teaches a approach by Deppe et al. that utilizes a different yet similar phase-locked surface-emitting laser diode array, which is fully disclosed an abstract document entitled “Phase-coupled two-dimensional AlxGa1-xAs/GaAs vertical-cavity surface-emitting laser array,” Applied Physics Letters, volume 56, pages 2089-2091, (1990). Wherein, they stopped the epitaxial growth of the vertical cavity with the upper spacer layer. After which, they formed a 2-μm wide Mn—Al metallization grid upon the top of the upper spacer layer and an insulating InP direct-bandgap semiconductor stack was deposited upon the top of the grid-covered spacer layer. Lasing, however, was not configured to occur beneath the grid.
Furthermore, phase-locked laser diode arrays, such as the one just described, present several unique applications. If the laser structures are phase locked with non-zero phase differences, the far-field intensity assumes a multi-lobed or at least off-axis pattern with the details of the patterns depending on the number of structures and the relative phase differences between each structure. If, however, the phase differences are controlled, then the intensity pattern can be controlled.
Consequently, the two-dimensional phased arrays of both EEL diode and VCSEL diode based designs have one major still unresolved flaw, due to the phase perturbation contributed by spontaneous emission, the phase difference exhibited by adjacent laser diode regions do not always equal zero, and the laser-emission-output for the majority of laser diodes comprising the array is out of phase (i.e., not phased locked) causing multiple lobes to appear in the far field pattern of a coupled laser-emission-output (i.e., a far field version of a multimode high-order transverse cavity mode intensity pattern). Resulting in a high degree of signal noise, which has keep these semiconductor laser diodes from being used in high-value applications; e.g., applications such as Telecommunications, Datacom, and the convergent “Passive Optical Networks” (PONs).
In addition, prior art also teaches a new kind of laser diode design approach altogether; currently, this laser diode design is called the “Quantum Cascade” (QC) laser diode, and was initially described in U.S. Pat. No. 5,457,709, incorporated herein by reference in its entirety. For more details, please see—U.S. Pat. No. 5,509,025; U.S. Pat. No. 5,901,168; and U.S. Pat. No. 6,055,257, which are altogether incorporated herein by reference in their entireties. Unlike the more conventional semiconductor laser diode, QC semiconductor lasers are unipolar, that is, they are based upon one type of carrier (i.e., typically electrons located in the conduction band), which make inter-subband transitions between energy levels that are created by quantum confinement. Further, in a unipolar semiconductor lasers, electronic transitions between conduction band states arise from size quantization made to occur in the active gain-region of a heterostructure. The inter-subband transitions are located between excited states of coupled quantum-wells for which, resonant tunneling is the pumping mechanism.
Furthermore, a single gain-region unipolar semiconductor laser is possible, but multiple gain-regions may be used as well. QC lasers comprise an active gain-region having a plurality (e.g., typically twenty-five) of essentially identical undoped laser-active semiconductor based layers, sometimes referred to as “Radiative Transition” (RT) regions. Each active (RT) region comprises as a plurality of semiconductor layers, and has quantum-well regions interleaved with barrier regions, to provide two or more coupled quantum-wells.
Moreover, prior art teaches that these coupled quantum-wells provide for at least second and third associated energy states for charged carriers (e.g., electrons). Further, the second energy state comprises of a lower energy than the third energy state, which corresponds to second and third wavefunctions, respectively. The energy difference between the third and the second energy state determines the wavelength of the laser-emission-output. The energy difference between second and third energy states is in turn determined by the arrangement of all the coupled quantum-wells in the active region. The arrangement includes the number of quantum-wells, the thickness of each individual quantum-well, the energy height, and thickness of each energy barrier layer that is located between two neighboring quantum-wells.
Furthermore, a multilayer carrier injector or injection region, sometimes referred to as an “injection/relaxation” (I/R) or “energy relaxation” region, is disposed between any two adjacent active regions. Thus, a given active region is separated from an adjoining one by an I/R region. The I/R region, like the active region, also typically comprises a plurality of semiconductor layers. Each active region-I/R region pair (i.e., each RT-I/R pair) may also be referred to as a ‘repeat unit.’ At least, some of the layers in each I/R region are doped, and in any case, the I/R regions, as well as the active regions, are unipolar.
The aforementioned U.S. Pat. No. 5,457,709, discloses a technique for designing a QC laser that uses the inter-subband transition located between energy levels of a coupled quantum-well structure and an I/R region with a digitally graded energy gap structure and the nominal structure of a QC laser is described in the aforementioned U.S. Pat. No. 5,509,025. Unlike a diode laser, the layers of a multilayer semiconductor QC laser structure are either undoped, or, if doped they are all of the same type, e.g. comprising an n-type dopant such as Silicon.
Moreover, an operating voltage is provided across the multilayer semiconductor structure of the QC laser. Further, this in turn causes substantial energy relaxation of charge carriers (e.g., electrons) in the I/R regions, some of which are introduced into the I/R region from an adjacent active-region. These injection carriers undergo a radiative transition, leading to lasing.
In addition, prior art further teaches many improvements to the QC semiconductor laser since its initial inception. For more details, please see—U.S. Pat. No. 5,457,709; and by J. Faist et al., “High Power Mid-infrared (λ0 about 5-μm) Quantum Cascade Lasers Operating Above Room Temperature,” published in Appl. Phys. Lett., vol. 68, pp. 3680-3682, (1996); and also see C. Sirtori et al., Appl. Phys. Lett., vol. 68, p. 1745, (1996). Moreover, for several types of applications, especially in the area of optical sensors configured for atmospheric trace gases, it is advantageous for the lasers to operate in a single transverse mode at a single frequency.
Furthermore, the use of distributed feedback (DFB) QC lasers for this purpose has been extensively explored, as described by J. Faist et al., “Distributed Feedback Quantum Cascade Lasers,” published in Appl. Phys. Lett., vol. 70, No. 20, pp. 2670-2672, (1997); by C. Gmachl et al., published in IEEE Photonics Technol. Lett., vol. 9, p. 1090, (1997); and by C. Gmachl et al., published in Appl. Phys. Lett., vol. 72, p. 1430, (1998).
Moreover, unlike other semiconductor lasers, such as laser diodes, the lasing wavelength of a QC semiconductor laser is essentially determined by quantum confinement (i.e., determined by the thickness of an active-region's layers), rather than by the bandgap of the active-region material. The lasing wavelength thus, can be tailored over a wider range than it can for a typical EEL diode using the same semiconductor material. For example, QC semiconductor lasers comprised with InAlAs/InGaAs active-regions have been tailored to operate at various mid-IR wavelengths ranging from 3.5-μm to 13-μm.
In addition, prior art also teaches that the use of diffraction gratings is one way to further control the operation frequency of semiconductor lasers. Further, the QC semiconductor laser can have its feedback provided by a DFB configuration, a DBR configuration, or a “Grating Coupled Surface Emitting Lasers” (GCSEL) configuration. For example, GCSELs are described in detail by A. J. Lowery, “Performance Comparison of Gain-Coupled and Index-Coupled DFB Semiconductor Lasers,” published IEEE J. Quantum Electronics, vol. 30, no. 9, pp. 2051-2063, (1994); by A. Kock, “Single-mode and Single-beam Emission from Surface Emitting Laser Diodes Based on Surface-mode Emission,” published in Appl. Phys. Lett., vol. 69 (24), pp. 3638-3640, (1996); by A. Rast et al., in “New Complex-Coupled DFB-Laser with a Contacted Surface Grating for λ=1.55-μm,” published in IEEE Proceedings Optoelectronics, vol. 142, no. 3, pp. 162-164, (1995).
Moreover, when using a diffraction grating, both the thickness of the active-region layers and the diffraction-grating determine the lasing wavelength, as follows. In a QC semiconductor laser, the characteristics of the active-region (i.e., the number and layer thicknesses of coupled quantum-wells) can be varied to determine the laser-emission wavelength range. The diffraction grating is used therein to precisely pick out a much narrower wavelength range within the range previously determined by the active-region layer thickness.
More succinctly, the grating controls the lasing wavelength more precisely than the lasing wavelength range determined by the layer thickness alone. However, the lasing wavelength selected by the diffraction grating cannot exceed the available laser wavelength range determined by the layer thickness and the whole laser structure. Thus, a unipolar injection laser, such as a QC laser, offers several advantages over bipolar semiconductor lasers. Compared to bipolar semiconductor lasers, these QC lasers have a frequency response not limited by electron/hole recombination, a narrow emission linewidth because the line-width enhancement factor is theoretically zero, and a weaker temperature dependence of the lasing threshold.
Additionally, as noted above, appropriately designed QC semiconductor lasers can have an emission wavelength in the spectral region from the mid-IR to the submillimeter region, which is entirely determined by quantum confinement. An alternative approach to fabricate a high-power, single-mode laser is to use a so-called ‘curved grating’. In such a laser, with a mode made up of counter-propagating diverging beams, the rulings of the diffraction gratings are curved to reflect one traveling wave into the other. See, e.g., Lang, R. J., “Design of Aberration-Corrected Curved Mirror and Curved-Grating Unstable-Resonator Diode Lasers,” IEEE J. Quantum Electron., vol. 30, p. 31 (1994). A method for fabricating the grating for a DFB semiconductor laser is disclosed in “Surface-Emitting Distributed Feedback Semiconductor Laser,” by S. Macomber et al., published in Appl. Phys. Lett. 51(7), pp. 472-474, (August 1987).
Moreover, this paper as prior art describes a technique in which a gold coating is deposited on a grating etched into the p-side of a semiconductor laser. The gold coating also serves as the p-contact for the laser. However, this approach employs an edge-emitting laser structure and has a large beam divergence angle (i.e., sometimes referred to as the diffraction angle) along the direction that is perpendicular to the laser surface, as well as a much higher optical power density at the laser facet that it is susceptible to catastrophic mirror damage. An edge-emitting semiconductor laser also typically has an elliptical, as opposed to circular, laser beam cross-section. This can require correction and collimating, which can be expensive or otherwise impracticable or undesirable.
In addition, prior art also teaches that due to the nature of the curved grating, the laser-emission-output beam has a spatial phase-difference distribution, which reduces optical beam quality. Many applications also require lasers that can operate in the mid-IR spectral range, (e.g., between 3-μm and 13-μm). Such applications might include:
Remote chemical sensing,
Pollution monitoring,
“Laser Infrared Detection and Ranging” (LIDAR),
Infrared counter-measure, and
Molecular spectroscopy.
Unfortunately, few convenient laser sources operate in the mid-IR spectral region. As noted above, for example, bipolar semiconductor laser diodes, including quantum-well laser diodes, have too large a bandgap, making it difficult, if not impossible, to obtain laser operation at mid-IR wavelengths. Some semiconductor laser diodes can operate in this wavelength range, but they require special cooling to a very low-temperature, which can be costly. QC lasers, however, as noted above, do not suffer these drawbacks, and can be designed to emit radiation at substantially any desired wavelength in a rather wide spectral region, including laser emissions in the mid-IR range.
Therefore, prior art further teaches that QC lasers are desirable for mid-IR range application. For example, QC lasers may be employed advantageously as radiation sources for the absorption spectroscopy of gases and pollutants, because at least some QC lasers can operate in the relevant wavelength region at or near room temperature, and with relatively high output power. For more details, please see—J. Faist et al., Applied Physics Lett., Vol. 68, pp. 3680-3682, (1996); and C. Sirtori et al., “Mid-infrared (8.5-μm) Semiconductor Lasers Operating at Room Temperature,” IEEE Photonic Technol. Lett., vol. 9 (3), pp. 294-296, (1997), both incorporated herein by reference in their entirety.
Moreover, prior art further shows that some applications require high-power laser-emission-output into a single fundamental transverse cavity mode. For example, for LIDAR, “Differential Absorption LIDAR” (DIAL), and other remote chemical sensing systems, a spatially coherent, single-mode, high-power laser-emission-output beam having the appropriate wavelength range can greatly increase the sensing range. Single-mode emissions from QC lasers can be achieved by incorporating a “Distributed Feed-Back” (DFB) or “Distributed Bragg Reflection” (DBR) reflection grating to form their respective resonating laser diode cavity structure.
It is difficult, however, to achieve a spatially coherent, single-mode, high-power laser-emission-output using conventional edge-emitting QC laser structures as the source of mid-IR radiation. To obtain high output power with an edge-emitting QC laser, one has to either use a very high injection current density into a narrow stripe laser configuration or use a broad area laser to increase the lasing area. A high injection current density will cause severe device heating, thus significantly limit the maximum laser-emission-output power, and therein, reduce the laser's lifetime.
Moreover, prior art further teaches that for any BALD device, regardless what type of semiconductor gain-region it might use, it is very difficult to maintain single mode operation under high-current injection operation because of the carrier induced refractive index change that causes the filamentation problem, which, as previously stated, hastens the onset of multi-moded lasing. For more details, please see—G. P. Agrawal & N. K. Dutta, “Semiconductor Lasers” (2nd edition; New York: Van Nostrand Reinhold, 1993).
Moreover, the self-induced filamentation effect produces a multi-spatial cavity mode laser-emission-output, which diminishes the laser's power and performance. The multi-mode laser emission is very difficult to focus, which is especially problematic when a long propagation distance or a very small focus beam size is required. Consequently, the multi-mode laser emission is less coherent and therefore, very difficult to use in several high-value applications such as Telecommunications.
In addition, prior art teaches that the output beam of an EEL diode has a very large divergence angle in a direction perpendicular to the laser diode's top surface. Surface emitting QC lasers, by contrast, show great promise for applications requiring stable higher-power laser-emission-output. A surface-emitting DFB based semiconductor laser diode has more potential to produce higher power laser-emission-output than does an EEL diode, because a larger lasing area can be used and the internal losses of the laser structure can be reduced.
Moreover, under current technology, the laser-emission-output from SELs can be spatially coherent if the width or the lateral dimension of the lasing region is limited to about 5-μm. To obtain higher laser-emission-output power, however, it is advantageous to provide a lasing gain-region with a width of 50-μm or more. Unfortunately, increasing the width of the gain-region typically leads to spatially incoherent operation at high current injection levels. Thus, there is a need for techniques for fabricating a surface emitting QC laser with both a wide lasing region and a spatially coherent laser-emission-output beam.
Prior art teaches several of these approaches, which have been proposed as a means to prevent the filamentation problem from occurring in a broad-area QC semiconductor lasers. Further, a typical solution is to create a so-called unstable resonance cavity (i.e., also referred to as an unstable resonant cavity or unstable resonator) within the laser diode device. There are several ways to create this kind of cavity for a high-power laser diode configuration.
Furthermore, a semiconductor laser diode with a continuous unstable resonator has been described by S. Guel-Sandoval et al., in a document entitled “Novel High-Power and Coherent Semiconductor Laser with a Unstable Resonator,” published in Appl. Phys. Lett., vol. 66, (1995), pp. 2048-2050, which is incorporated herein by reference in its entirety. Further, this paper fully describes a means for inducing a quadratically varying index of refraction across the lateral dimension of a wide-stripe semiconductor laser diode, in order to introduce some control over beam divergence and the coherent operation of the laser diode device.
In addition, prior art also teaches an approach, which uses the curved grating described in the Lang reference above. Further, a new way to fabricate a grating-coupled surface-emitting laser diodes with an unstable resonance cavity is disclosed in aforementioned U.S. Pat. No. 5,727,016. This patent describes the use of a variable index refraction layer, having an approximately parabolic-trough, therein. The refraction layer is positioned adjacent to the active lasing region. A straight-toothed, second-order diffraction grating contacts the refraction layer to produce a broad, spatially coherent output beam. For more details, please see—A. Kastalsky, “Infrared Intraband Laser Induced in a Multiple-Quantum-Well Interband Laser,” IEEE J. Quantum Electronics, vol. 29, no. 4, pp. 1112-1115, (1993).
Therefore, prior art teaches that there is a need for improved surface-emitting QC lasers, which produce high-power, spatially coherent, single-mode output beams, having a small divergence angle. Such devices especially would be useful, for example, for remote chemical sensing and LIDAR applications. A compact, low-cost, and reliable high-power, spatially coherent, single-mode, mid-IR semiconductor laser can greatly reduce the system cost and reliability for these applications.
Moreover, as prior art has proposed, a QC laser that incorporates grating-coupled, surface-emitting, and unstable resonance cavity structures would find application in technologies needing a mid-IR coherent light source. The grating-coupled, surface-emitting structure of the present invention provides the advantage of a narrow spectral width laser output with a small divergence angle, and the unstable resonance cavity structure provides the advantage of preventing the filamentation effect that causes multimode lasing under high injection current. The combination of these two structures allows the laser to maintain narrow spectral, single mode, and small diffraction output at high injection current. This combination in a unipolar QC laser also allows lasing emissions to be achieved over a wide spectral region, including wavelengths in the mid-IR range.
In addition, prior art also teaches that because it is surface emitting, the GCSEL exhibits a circular laser-emission-output beam and a smaller divergence angle, and can therefore, be more attractive than EEL diodes in some applications. A typical QC laser is of unipolar semiconductor laser design having a multilayer stacked structure epitaxially grown upon a semiconductor substrate, typically Indium-Phosphide, which forms an optical waveguide structure therein. The optical waveguide structure includes a core or active-region of relatively large effective refractive index (e.g., an index of 3.3) between cladding-regions of relatively small effective refractive index (e.g., an index of 3.1). Further, a cladding-region will also be referred to herein, as a ‘confinement region’.
Moreover, prior art further teaches that the core-region comprises a plurality of repeat units, each unit having essentially an identical active-region (i.e., sometimes called the gain-region or gain-medium), and a carrier injection/relaxation (I/R) region, as described above. The core-region generates lasing when electrical power is applied to the structure through electrodes. The lasing light propagates within the optical waveguide, which includes the core-region, in the longitudinal direction of the cavity, where it is amplified by the lasing action.
Moreover, prior art further teaches that the nominal structure of this aspect of the QC laser is similar to that described by the aforementioned U.S. Pat. No. 5,509,025. As noted above, diffraction gratings may be used to control the operational frequency of semiconductor laser diodes by helping to select a narrower lasing frequency range within the frequency range set by the laser diode structure itself. Grating coupled DFB structures, for example, have been applied to the basic QC laser structure to produce single-mode lasers having a pre-selected wavelength. In addition, a QC laser structure that uses a DFB configuration will only utilize a first order grating.
For example, prior art teaches that a first order grating may be used to couple light in the longitudinal direction of the laser cavity (i.e., edge emitting). Gratings can also be made for the use of higher coupling orders, as will be appreciated by those skilled in the art. To fabricate a SEL QC laser, for example, one would need to incorporate a second order grating into a laser structure to produce a grating coupled SEL. Further, laser diodes with grating coupled surface emitting structures are well known. See, for instance, the aforementioned Kock reference, which discloses a surface-emitting bipolar laser diode with a second-order grating in the top-cladding layer, the grating facilitated coupling of the laser cavity mode into a surface mode. A second order grating causes vertical coupling, instead of in the cavity longitudinal direction.
Regardless, prior art further teaches that the main advantage of a surface coupled laser is that the laser-emission-output beam has a much smaller divergence angle than a conventional EEL diode, and can be circular in cross section rather than elliptical, as is the case for EEL diodes. A laser-emission-output beam having a smaller divergence angle and circular cross section is easier to focus into a smaller spot size or collimate into a laser beam that can maintain a smaller spot size after the beam has traveled a long distance, and easier to couple into a fiber or other light-receiving device.
Moreover, the QC laser of the present invention employs a grating-coupled, surface-emitting structure, to provide a narrow spectral linewidth (e.g., MHz) laser output having a small divergence angle (e.g., 1°). In conclusion, while prior art shows conclusively that QC semiconductor lasers can made somewhat stable, they still suffer from the same phase perturbation problem contributed by spontaneous emission that all other semiconductor lasers suffer from regardless any improvement.
In addition, prior art also teaches how and why conventional resonators control transverse spatial cavity modes, both for macroscopic and microscopic optical systems. Further, at scales associated with microscopic optical systems, which include single mode optical fiber, semiconductor gain-media, and “Micro-Opto-Electro-Mechanical-System” (MOEMS) devices, transverse spatial cavity mode control can dictate many system design variables.
Typically, the low-order fundamental transverse mode operation is desired in laser diode devices, this is due to the optical beam spatial profile requirements for long distance beam propagation, the focusing of laser-emission-output beams into small spots, and laser-emission-output beam coupling into single mode transmission fibers.
In addition, prior art also teaches that the different transverse spatial cavity modes of an optical resonator will typically have different resonant optical frequencies, which is characteristically detrimental for both active and passive cavity applications requiring spectral purity. A typical application requiring spectral purity of resonator operation is the application of spectral monitoring of the optical communications signals being manipulated by “Wavelength-DivisionMultiplexed” (WDM) optical transmission equipment, using tunable Fabry-Perot filters.
Moreover, prior art teaches that for active cavity devices, such as EEL semiconductor laser diodes, the transverse spatial cavity mode problem is addressed by the judicious design of the laser waveguide to ensure that it supports only a single fundamental transverse cavity mode.
In addition, prior art teaches that for VCSELs, oxide confining layers, and other aperturing techniques are used to achieve single transverse cavity mode operation in small aperture devices. Problems begin to arise, however, when we try to design high-powered laser-emission-output capable VCSELs; wherein, a contradictory contention arises between the desire to increase modal volume and beam diameter size, and the desire to suppress oscillation of the higher-order transverse cavity modes.
Typically, an oxymoron for designers and engineers alike. Further, prior art teaches that for passive cavity devices, the transverse spatial mode problem is more intractable, since the high-degree of freedom associated with the design of the gain-medium is not present. One solution is to incorporate a single mode optical fiber into the design. The inclusion of optical fiber, however, tends to complicate device integration, creates fiber-coupling requirements, and does not resolve all of the spatial cavity mode problems present.
In addition, prior art also teaches a related solution to controlling the transverse “Side Mode Suppression Ratio” (SMSR), which contemplates the use of intracavity apertures or spatial filters. Further, higher-order spatial cavity modes generally have larger mode field diameters than do the lower-order fundamental transverse modes such as TEM00. Wherein, apertures in an optical train can induce loss for the higher-order transverse cavity modes, and may be used to improve the laser diode's side-mode suppression. These spatial filters, however, can also introduce loss for the fundamental transverse spatial modes as well, and therefore require precise alignment.
In addition, prior art teaches another solution concerning cavity design. Whereby, in a confocal Fabry-Perot cavity, where cavity length is equal to the mirror radius of curvature, all trans-verse modes are degenerate, i.e. all the transverse spatial modes coexist on the same frequencies, or wavelengths, as the longitudinal mode frequencies, or the longitudinal mode frequencies shifted by a half spectral period.
Furthermore, MOEMS comprise micro-optical cavities that typically have large free spectral ranges, or spectral periods, corresponding to small cavity lengths of only tens of micrometers. Therefore, as prior art suggests, a confocal MOEMS micro-cavity configuration would require mirrors with correspondingly small radii of curvature (i.e., tens of micrometers), which are difficult to fabricate and have small cavity mode sizes, which are also difficult to align. Further, a more typical, and probably a more feasible configuration used by MOEMS would be a tunable-filter based Fabry-Perot cavity that utilizes a hemispherical cavity configuration.
Moreover, in such cavities, one of two reflectors is near planar and the other reflector is a spherical in shape. The advantage here is reduced alignment criticalities; this is because of the general radial homogeneity contributed by the near planar reflector. Further, in such configurations, spatial mode spectral degeneracy is not present, and any higher-order transverse cavity modes present themselves as only a few spurious peaks, which are observed in the filter transmission spectrum.
Moreover, prior art further teaches that these problems have led to solutions that focus on minimizing the excitation of higher-order modes by precise control of how light is launched into the laser diode's cavity. For example, please see—U.S. patent application Ser. No. 09/666,194, filed on 21 Sep., (2000), by Jeffrey A. Korn, and U.S. patent application Ser. No. 09/747,580, filed 22 Dec., (2000), by Walid A. Atia et al., which are disclosures that concern, in part, an alignment of a tunable filter relative to the surrounding optical train.
For more details, please see—U.S. patent application Ser. No. 09/809,667, filed on the 15 of Mar. (2001), by Jeffrey A. Korn, which disclosures, in part, a mode field matching between the launch light mode and the lowest-order spatial transverse cavity mode of the filter. Further, such design approaches minimize excitation of higher-order transverse spatial cavity modes and thus, yield systems with better side-mode suppression ratios.
In conclusion, prior art clearly shows that the SEL based semiconductor laser diodes as described above, as described in the included materials, and mentions, while in some cases might provide for low-order fundamental transverse spatial cavity mode laser-emission-output, they all typically suffer from three major performance issues:
Because they do not neutralize the phase perturbations contributed by spontaneous emission to resonance, the laser-emission-output remains unstable manifesting multiple instabilities (e.g., spatial mode hopping, spectral mode hopping, polarity switching, high threshold currents, filamentation, multi-transverse operation at threshold, multi-longitudinal operation at threshold, spatial hole burning, spectral hole burning, low-power laser-emission-output, overall degradation of laser diode performance, and single low-order fundamental transverse spatial cavity mode being virtually un-attainable over a wide range of power levels;
Because the solutions described above and in the included references focus on introducing loss for high-order transverse spatial cavity modes, the overall power performance of the laser diode is seriously degraded to the point where these laser diodes only produce around 2-mW to 3-mW of laser-emission-output power and therefore, not applicable for most current high-valued application; e.g., applications such as optical Telecommunication and Data transmission; and
Because these devices (mostly EELs) suffer from intra-cavity pulse-broadened dispersion (i.e., chirp) they have reached their internal limits for data modulation and therefore, can not transmit data any faster than just a few Gigabits/ps. Consequently, this limits them to low-valued applications; e.g., such as multimode fiber optic LANs, fiber channel interconnects, and other non-proprietary Datacom and Enterprise based application.
Consequently, my OPCLD invention, by using an optical “Phase Conjugation Mirror” (PCM) in place of the more conventional “Cleaved Facet” (CF), “Distributed Bragg Reflection” (DBR), and “Distributed Feed-Back” (DFB) mirror designs typically used by VCSELs, VECSELs, EELs, and BAID devices, it successfully neutralizes phase perturbations that being the root cause of performance degradation and the laser-emission-output instabilities that regularly manifest themselves in all current semiconductor laser diodes as higher threshold currents, low-power laser-emission-output, unstable spatial cavity modes, unstable spectral cavity modes, instability of polarity for laser-emission-output, filamentation, and COD.